Image state linewidth on Ag(100) studied by two-photon photoemission spectroscopy

The linewidth of the first image state on Ag(100) was studied using two-photon photoemission spectroscopy. The width was found to be 35±9 meV in agreement with a recent time-resolved study (Schoenlein et al.: Phys. Rev. Lett.61, 2596 (1988)) but somewhat larger than the theoretical value of 10–20 meV (Echenique et al.: Phys. Rev. B35, 4529 (1987)).     

Momentum-resolved lifetimes of image-potential states on Ag(001)

Binding energies and decay rates of image potential states at an Ag(001) surface have been investigated with time-resolved two-photon photoemission. For the first four image potential states the binding energies are determined to be 561, 170, 72 and 37 meV with respect to the vacuum level. Lifetimes of the first three states are extracted to be 57, 165 and 380 fs at k_∥ = 0. With increasing parallel momentum k_∥ the lifetime of the n = 1 state decreases such that the inverse lifetime scales proportional to the kinetic energy within the parallel motion with 34 meV/eV. As light source for the pump-probe photoemission experiments a novel all fiber based femtosecond laser system with a repetition rate of 1.5 MHz has been used.     

Spin-dependent electron dynamics in front of a ferromagnetic surface

Exchange splitting and dynamics of image-potential states in front of a 3 monolayer iron film on Cu(100) have been studied with time-, energy-, and spin-resolved bichromatic two-photon photoemission. For the first image-potential state n=1 we observe an exchange splitting of 56 +/- 10 meV and spin-dependent lifetimes of 16 +/- 2 fs for majority-spin and of 11 +/- 2 fs for minority-spin electrons, respectively. The time-resolved studies of both the population and the linewidth of image-potential states manifest that at the magnetic surface not only inelastic but also quasielastic scattering processes are spin dependent.     

Electronic structure and electron dynamics at Si(100)

The electronic structure and electron dynamics at a Si(100) surface is studied by two-photon photoemission (2PPE). At 90 K the occupied D_up dangling-bond state is located 150±50 meV below the valence-band maximum (VBM) at the center of the surface Brillouin zone ̄ and exhibits an effective hole mass of (0.5±0.15)m_e. The unoccupied D_down band has a local minimum at ̄ at 650±50 meV above the VBM and shows strong dispersion along the dimer rows of the c(4×2) reconstructed surface. At 300 K the D_down position shifts comparable to the Si conduction-band minimum by 40 meV to lower energies but the dispersion of the dangling-bond states is independent of temperature. The surface band bending for p-doped silicon is less than 30 meV, while acceptor-type defects cause significant and preparation-dependent band bending on n-doped samples. 2PPE spectra of Si(100) are dominated by interband transitions between the occupied and unoccupied surface states and emission out of transiently and permanently charged surface defects. Including electron–hole interaction in many-body calculations of the quasi-particle band structure leads us to assign a dangling-bond split-off state to a quasi-one-dimensional surface exciton with a binding energy of 130 meV. Electrons resonantly excited to the unoccupied D_down dangling-bond band with an excess energy of about 350 meV need 1.5±0.2 ps to scatter via phonon emission to the band bottom at ̄ and relax within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds. PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

Electronic structure of the molecular switch tetra-<Emphasis Type="Italic">tert</Emphasis>-butyl-azobenzene adsorbed on Ag(111)

Occupied and unoccupied electronic states in tetra-tert-butyl-azobenzene (TBA) absorbed on Ag(111) have been investigated by one-photon and two-photon photoemission spectroscopy. These measurements allow the quantitative determination of energetic positions of the highest occupied (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as the n=1 image potential state. The assignment of the electronic states are supported by quantum chemical calculations. Experimentally a HOMO–LUMO gap of 2.85 eV is observed, whereas the gap obtained from the calculated molecular orbital energies is 0.92 eV larger. This discrepancy can be explained by image charge screening. Furthermore, two unoccupied final states located 0.18 and 0.43 eV above the vacuum level, respectively, have been identified. PACS 73.20.-r; 74.25.Jb; 79.60.-i; 79.60.Dp; 68.43.Vx     

Unoccupied surface states on the (111) surface of silver: Evidence for broadening

We have studied Ag(111) withk-resolved inverse photoemission spectroscopy athv=9.7 eV. In normal incidence we find image-state emission atE _vac–(0.4±0.1) eV and the unoccupied part of an intrinsic surface-state band as a huge emission peak cut byE _F. The energy dispersion of the intrinsic surface-state band and in particular its crossing ofE _F predicted by Ho et al. cannot be observed because of broadening effects as is shown by a theoretical simulation. The broadening is due to the vicinity of the surface state to the bulk continuum nearE _F as suggested by Kevan.     

Spectroscopy of image-potential states by two-photon photoemission

Unoccupied electronic states in solids and at solid surfaces are usually studied by inverse photoemission. An alternative method is two-photon photoemission. It is superior in resolution but limited to states of sufficiently long lifetime below the vacuum level. So far this method has mainly been applied to image-potential states on metal surfaces. On Ag(111) and Cu(111) a narrow surface state below the Fermi level serves as the initial state, which results in a pronounced resonance in the two-photon photoemission. Ni(111) shows similar results. In the resonance the image-potential state is so highly populated that electron-electron interaction leads to an Auger-type process. Nevertheless, the system is not so greatly disturbed as to show deviations from the one-photon photoemission results concerning the occupied states. Ag(100) and Cu(100) have a smooth continuum of initial states. Consequently, no resonance occurs. The binding energy does not depend on the material but changes with surface orientation: it is about 0.80 eV at the (111) surfaces and about 0.55 eV at the (100) surfaces. The effective mass is free electron like except on Ag(111), where it is 30% heavier. The lifetime on Ag(100) is about 20 fs. The agreement with theory is excellent in some cases and only fair in others.     

Quantization of electronic states on metal surfaces

An electron in front of a metal surface experiences an attractive force due to the induced image charge. Band gaps in the band structure can prevent a penetration into the metal along certain directions. The Coulomb-like potential supports bound states in front of the surface which correspond to a hydrogen atom in one dimension. These image states can be measured with high resolution by two-photon photoemission. The adsorption of metals modifies the states. If the electrons can penetrate into the metal, quantum-well states can develop corresponding to standing waves in the overlayer. Image states on small islands show the quantization effects due to the lateral localization. The spectroscopy of image states by two-photon photoemission permits the investigation of growth and morphology of deposited metal layers, a well as the illustration of fundamental quantum-mechanical effects.     

Photoinduced interface charging in multiphoton photoemission from ultrathin Ag films on Si(100)

The fluence dependence of two-photon photoemission and time resolved two-color pump–probe photoemission spectroscopy of a 25 ML thick Ag-film grown on n-doped Si(100) reveal a photoinduced work function reduction that is attributed to a reduction of the surface dipole. Time-resolved two-color pump–probe spectroscopy shows that this reduction persists for at least several microseconds. This and the pump-induced modification of the 4.65 eV two-photon photoemission spectrum indicate that the excitation of long-lived trap states at the Ag-Si interface affects the charge distribution in the Ag film and consequently is responsible for the reduction of the surface dipole. PACS 79.60.Bm; 79.60.-i; 82.50.Nd, 73.50.-h     

Structural transition in cyclooctatetraene adsorbed on Ru(0 0 1) probed by thermal desorption and two-photon photoemission spectroscopy

The adsorption of 1,3,5,7-cyclooctatetraene (COT) on Ru(0 0 1) is studied by temperature-programmed desorption (TPD), work function measurements, as well as time- and angle-resolved two-photon photoemission (2PPE) spectroscopy. The TPD data show that COT films grow at 115 K in a metastable phase when the coverage is increased from the chemisorbed monolayer to the bulk-like molecular multilayer structure. The metastable states desorb at a temperature which is ≈9 K lower than the desorption temperature of the stable multilayer. At 165 ± 2 K, they undergo an irreversible and thermally activated transformation into the stable multilayer phase. This transition is accompanied by a pronounced increase in the total 2PPE yield by more than one order of magnitude as well as the appearance of image potential states. The image states have binding energies of −0.70 eV and −0.24 eV for the n = 1 and n = 2 states, respectively, and a lifetime of 20 ± 5 fs for both states. Their appearance is interpreted as an indication of island formation in the stable multilayer regime. 2PPE spectroscopy of the image potential states provides a sensitive probe of structural transitions in the adsorbate layers.     

Tunneling measurements on (La1−x Ce x )Al2 single crystals with wax barriers

In the investigation of the Kondo superconductor (La_1–x Ce _x )Al₂ a new technique is developed to fabricate tunnel junctions of the type: bulk sample — wax barrier — evaporated counterelectrode. Pb, Al, Ag, Au and Mg are used as counterelectrodes. Measurements employing Pb and Al yield the order parameter of nominally pure LaAl₂: ₀=(0.47±0.03) meV=1.63kT _c. The large influence of localized states within the gap is confirmed.     

Femtosecond two-photon photoemission at 150 kHz utilizing two noncollinear optical parametric amplifiers for measuring ultrafast electron dynamics

An improved setup for femtosecond two-photon photoemission spectroscopy (TR-2PPE) is presented. Two noncollinear optical parametric amplifiers (NOPA) were operated simultaneously at a repetition rate of 150 kHz. The frequencies of the NOPA outputs were tuned such that subsequent second harmonic generation (SHG) provided the two different ultraviolet wavelengths required for the pump and probe pulse. The width of the crosscorrelation function (CC) for pump and probe pulse was sub-30 fs (FWHM).The performance of this assembly was tested by measuring the lifetime of image potential states (IS) on Cu(111) and Ag(111) surfaces. We present here for the first time lifetimes of the IS that were determined directly from the measured decay of the 2PPE signal.This revised version was published online in August 2005 with a corrected cover date.     

Electronic properties of (Co, Ag) self-organized nano dots on Au(1 1 1) vicinal surfaces

Electronic properties of (Ag, Co) nanostructures grown on Au(1 1 1) vicinal surfaces have been studied by angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy and spectroscopy (STM/STS). The growth and self-assembling of Co and Ag nano dots on Au(7 8 8) surface are described. Co island growth leads to the formation of repulsive energy barriers for the surface state, and subsequently to the appearance of confined states in between each group of four Co dots. On the contrary, when Ag nano dots are grown, the potential barrier for the surface electrons is not enough to suppress their dispersive behavior. Nevertheless, inside Ag islands appear new quantized states whose energies can be tailored by varying the deposition rate of the adsorbate and/or the Miller index of the vicinal surfaces. In both systems, high homogeneity of the electronic properties is achieved over a macroscopic scale.     

Photoemission from multiply excited surface plasmons in Ag nanoparticles

Two-photon photoemission spectroscopy using femtosecond laser pulses is used to investigate the excitation and decay mechanisms of the surface plasmon resonance in Ag nanoparticles grown on graphite. The resonant excitation of this collective excitation leads to a two-orders-of-magnitude-enhanced two-photon photoemission yield from a graphite surface with Ag nanoparticles compared to the yield from pure graphite. From the shape of the photoemission spectra, the polarization dependence of the photoemission yield and the excitation probabilities for different excitation pathways we conclude that excitation with 400-nm femtosecond laser pulses leads to the coherent multiple excitation of the surface plasmon in the Ag nanoparticles. This multiply excited plasmon mode can decay via the coupling to a single-particle excitation leading to the emission of an electron if its final state is located in the continuum. The surface plasmon in metallic nanoparticles is a model system to investigate collective excitations in multiphoton processes. Received: 26 June 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Modelling of interferometric multiphoton photoemission

A new interferometric multi-photon photoemission scheme has been recently introduced by Güdde et al. [1] and applied to a study of the n=1 image state on Cu(100). We report on the results of a theoretical modelling of this photoemission scheme based on a wave function representation. It incorporates both resonant transitions and two-photon non-resonant transitions. In the case of the n=1 image state on Cu(100), the initial state of the photoemission belongs to a continuum. Summation of the contributions to the photoemission yield from the various initial states is shown to drastically blur the interference structure, acting as an efficient dephasing process. The sensitivity of the new scheme and of the one-colour TR-2PPE scheme to the system dynamical properties is discussed. PACS 73.20.-r; 78.47.+p; 79.60.-i; 82.53.Kp; 42.50.Md     

Transient fields for Ag and Pd ions IN Fe and Co;g-Factor measurements of the lowest 3/2− and 5/2− states in107,109 Ag

Transient field precessions of the 2 ₊ ¹ state of¹⁰⁸Pd were measured simultaneously with those of the 3/2 _– ¹ and 5/2 _– ¹ states of¹⁰⁷Ag and¹⁰⁹Ag as their ions traversed thin Co and Fe foils. Precessions of the 2 ₊ ¹ states of^{104,106,108,110}Pd were also measured using a natural Pd target evaporated onto 0.9 m thick Co. The results of these studies are in accord with prior predictions that the AgFe transient field might be diminished relative to that for PdFe at ion velocities appropriate for molecular orbital vacancy sharing. Transient field strengths for PdCo and PdFe were found to be consistent with scaling according to bulk magnetization of the ferromagnetic host. Gyromagnetic ratios were inferred for the first 3/2^– and 5/2^– states of^107,109Ag relative to theg factor of the 2 ₊ ¹ state of¹⁰⁸Pd. Values obtained usingg(2 ₊ ¹ ;¹⁰⁸Pd)=0.36± 0.03 are:g(3/2;¹⁰⁷Ag) =0.63±0.09,g(5/2;¹⁰⁷Ag)=0.37±0.06,g(3/2;¹⁰⁹Ag)=0.77 ±0.10,g(5/2;¹⁰⁹Ag)=0.36±0.05.     

Optical intersubband transitions and femtosecond dynamics in Ag/Fe(100) quantum wells

The optical intersubband transitions and femtosecond dynamics of electrons in quantum well states in Ag/Fe(100) are investigated by interferometric time-resolved two-photon photoemission. The quantum well wave functions and transition probabilities are evaluated from the two-photon photoemission resonance energies and intensities using an extended phase accumulation model. Direct femtosecond pump-probe correlation measurements elucidate the importance of interfaces in confined structures.     

Image-potential states on stepped Cu (001) surfaces

Image-potential states on Cu (117) and Cu (119) surfaces were studied by means of two-photon photoelectron spectroscopy. The regular array of steps generates a lateral potential on the vicinal surfaces, which modifies the surface-electronic structure. Compared to Cu (001), the band bottom of the n=1 image-potential states shifts by 40 meV to lower binding energy. The periodicity of the step-induced superlattice manifests itself as back-folding of the n=1 and 2 dispersion bands. At the surface Brillouin zone boundary a mini-gap opens with a width of 135 meV for the first image-potential state on Cu (117). On the vicinal surfaces the lifetime of the image-potential states is reduced by a factor of three as compared to Cu (001). This is attributed to a narrowing of the surface-projected bulk-band gap when projected along the [11n] direction. While the dephasing rate of the first image-potential state is close to the decay rate, higher members of the Rydberg-like series show negligible dephasing. Received: 16 October 2001 / Revised version: 9 April 2002 / Published online: 6 June 2002     

Scattering of Cu(1 0 0) image state electrons from single Cu adatoms and vacancies: A comparative study

A theoretical study of the electron dynamics in image potential states on Cu(1 0 0) surfaces with different types of defects (Cu adatoms and Cu vacancies) is presented for low defect density at the surface. A wave packet propagation approach is employed for the electron scattering calculations, where the defect induced potentials are obtained from an ab initio density functional study. Scattering of the image state electron by a defect induces inter-band and intra-band transitions leading, respectively, to the population decay and to the dephasing of the image states. Comparison of the respective effects of adatoms and vacancies shows that Cu adatoms are much more efficient in inducing population decay and dephasing of the image potential states. Present results for the case of Cu adatoms are compared with available time-resolved two-photon photoemission data.     

Surface states on random substitutional alloys

We present first principles KKRCPA calculations of the angle-resolved photoemission spectra from CuNi single crystal surfaces. Two surface states have been identified; a Shockley-type state on the (111) surface, and a Tamm-type state on the (100) surface. We determine their dispersion, and investigate their sensitivity to the form of the surface potential. The disorder broadening and the influence of surface segregation are discussed quantitatively.